[ci] invert the condition on all-jobs-succeeded

Previously, a failing job could cause all-jobs-succeeded to be
skipped, and branch protections interpreted a skipped job as
being vacuously successful. This failure mode caused #454 to be
merged to main, despite failing many CI checks.

This PR inverts the `if` condition of this job: It only runs on
failure, and upon failure, it fails.

.github/workflows/ci.yml

@@ -443,11 +443,11 @@ jobs:
  cargo kani setup &> /dev/null || true
 
  # Used to signal to branch protections that all other jobs have succeeded.
- all-jobs-succeed:
- name: All checks succeeded
- if: success()
+ no-jobs-failed:
+ name: No checks failed
+ if: failure()
  runs-on: ubuntu-latest
  needs: [build_test, kani, check_fmt, check_readme, check_msrv, check_versions, generate_cache]
  steps:
- - name: Mark the job as successful
- run: exit 0
+ - name: Mark the job as unsuccessful
+ run: exit 1